1. Field of the Invention
The invention relates generally to the field of ocean bottom cable (OBC) seismic survey sensor systems. More specifically, the invention is related to devices for improving the efficiency of deployment and retrieval of OBCs, and for reducing the incidence of damage to OBCs during deployment and retrieval and during periods of operation in high sea states.
2. Background Art
Seismic surveying performed in bodies of water (marine seismic surveying), such as lakes or the ocean, includes ocean bottom cable (OBC) surveying techniques. OBCs are deployed on the water bottom and include seismic sensors arranged such that various techniques may be applied to the sensor measurements to attenuate undesirable artifacts common to marine seismic data known generally as “ghosts” and “water layer multiples.”
An OBC includes a length of reinforced electrical and/or optical cable having sensors, or sensor “units” coupled with or disposed along the cable at spaced apart locations. The sensor units typically include one or more particle motion sensors, such as geophones or accelerometers, and at least one sensor responsive to pressure (or a sensor responsive to rate of change of pressure). Electrical conductors and/or optical fibers in the cable conduct signals from the various sensors to a recording device typically coupled to one end of the cable.
OBCs are typically deployed by unspooling them from a winch drum or reel located on a deployment vessel (called a “cable handling vessel”), allowing the cable with a suspended weight to reach the bottom of the body of water. The cable handling vessel moves in a direction along which it is intended to position the OBC along the water bottom for a seismic survey and slowly deploys the cable to maintain a predetermined critical tension. When the OBC is laid to an extent such that the nearest sensor unit (that sensor unit closest to the cable handling vessel) reaches the water bottom under the proper tension, the cable handling vessel is typically stopped. The un-spooling continues until a sufficient additional length of lead in cable is laid on the bottom so as to compensate for buoy/vessel drift, high seas or the collection of multiple OBC lead-ins by a single recording vessel. This continued un-spooling will typically create loops in the lead in cable After unspooling is completed, a buoy or similar device may be attached to the water surface end of the OBC, such that a recording system may be coupled to the OBC for subsequent seismic data acquisition and recording. The recording system may, alternatively, be located on the recording vessel such that buoy connection is not required.
It will be appreciated by those skilled in the art that a “lead in” is a section of cable or cables, depending on water depth, having no sensors and connecting the sensor array (OBC) to either a cable handling vessel, a recording vessel or a buoy. After unspooling is completed, the lead-in is under only the tension resulting from the lead in cable weight in the water column, with the loops in the lead in cable allowing for the OBC sensor cable to remain in place and unaffected by sea states or other high tension transient loads that would otherwise cause the cable to move off the specified cable position on the water bottom or to torque and create through a loop in itself.
OBCs made for relatively shallow water may include a centrally disposed electrical conductor surrounded by a layer of insulation. The insulation may then be surrounded by an electrically conductive metal braid, which in combination with the central conductor serves as a coaxial cable. The exterior of the OBC is typically surrounded by a plastic jacket to exclude water and to provide electrical insulation. In such shallow water OBCs, there may be one or more reinforcement layers to provide axial strength to the OBC. Typically, in such OBCs the reinforcement layer is in the form of a woven fiber braid. Such shallow water OBCs, having only braided reinforcement devices, are substantially free of induced torque when tension on the cable is increased and decreased. Deployment of such OBCs is not typically associated with any difficulties relating to torque along the cable caused by tension. However, there is a tendency of such shallower depth OBCs to assume the shape of the winch or reel while on the winch and under tension. As tension is relieved during deployment, the OBC may form loops where the OBC tries to return to its shape under tension. Such loops will not be relieved or unwound as the OBC is retrieved from the water bottom. In such cases, the loops may cause the OBC to kink when tension is reapplied. Kinking may damage the cable, thus necessitating expensive repair or replacement of the cable and shutting down the operation.
As OBCs are made to be used in deeper bodies of water, it has proven necessary to use cable structures that have various forms of wound wire armor, in order that the cable will have sufficient axial strength to support its own weight when suspended in the body of water. For example, in a typical OBC used for water depths of 3,000 meters or less, the cable may include three, concentrically placed, helically wound layers of armor wires surrounding the center conductor and shield layer. When helically wound armor wires are subjected to axial stress, they impart a torque to the cable as they tend to unwind. While typical armored electrical cables include a plurality of contrahelically-wound layers of armor wires (meaning that successive layers are wound with opposing helical lay direction), it is impracticable to create a completely torque balanced, wound wire armored cable. Torque balanced in this context means that there is substantially no torque along the cable within a specific range of cable loads. In the foregoing example of a deeper water OBC, as the lead in is deployed, substantially all of the axial stress is relieved at the water bottom position of the lead in. Such stress relief generates substantial torque imbalance along the cable at the water bottom and at the water column interface. Due to operational conditions an additional amount of lead-in cable must be ‘dropped in-loops’ on the water bottom to allow for extension from the buoy's interaction with the sea surface during rough sea states, or with ‘rolling’ of the cable to connect with the recording vessel as described below. As the cable is pulled and relaxed by motion of the buoy or vessel at the surface, the loops are pulled off the water bottom and are pulled into tight loops or kinks. The loops can cause the cable to kink when the cable is retrieved from the water bottom.
In multiple-cable OBC surveys, a plurality of OBCs are typically deployed on the water bottom substantially parallel to each other along a selected direction. Each OBC in the multiple-cable survey includes a lead in made substantially as described above for a single cable OBC survey. In a multiple cable OBC survey, however, the lead in is typically terminated at a common location at the water surface. During a multiple cable survey, a recording vessel is connected to the water surface ends of all the OBCs. During the survey, a laterally endmost one of the OBCs is disconnected from the surface location, and the recording vessel is moved laterally while still connected to several of the remaining OBCs. The disconnected OBC is retrieved by the deployment vessel and may be moved to a location along the opposed lateral end of the “spread” of OBCs on the water bottom. Lateral movement of the recording vessel imparts lateral tension along the connected OBCs and causes the cable to ‘roll’ along the water bottom Such lateral movement is another source of torque which may result in kinks in the OBCs. Just as in the case of the single OBC survey operation, when an OBC having loops therein is retrieved, the rapid application of axial stress may result in kinks in the cable as the torque along the loop cannot be quickly relieved.
It is desirable to have a system for OBC surveying which reduces the possibility of looping and consequent kinking in the cable.